----- On Oct 26, 2020, at 4:53 PM, paulmck paul...@kernel.org wrote: > On Mon, Oct 26, 2020 at 03:58:11PM -0400, Mathieu Desnoyers wrote: >> ----- On Oct 22, 2020, at 6:30 PM, paulmck paul...@kernel.org wrote: >> >> > The current code can lose RCU callbacks at shutdown time, which can >> > result in hangs. This lossage can happen as follows: >> > >> > o A thread invokes call_rcu_data_free(), which executes up through >> > the wake_call_rcu_thread(). At this point, the call_rcu_data >> > structure has been drained of callbacks, but is still on the >> > call_rcu_data_list. Note that this thread does not hold the >> > call_rcu_mutex. >> > >> > o Another thread invokes rcu_barrier(), which traverses the >> > call_rcu_data_list under the protection of call_rcu_mutex, >> > a list which still includes the above newly drained structure. >> > This thread therefore adds a callback to the newly drained >> > call_rcu_data structure. It then releases call_rcu_mutex and >> > enters a mystifying loop that does futex stuff. >> > >> > o The first thread finishes executing call_rcu_data_free(), >> > which acquires call_rcu_mutex just long enough to remove the >> > newly drained call_rcu_data structure from call_rcu_data_list. >> > Which causes one of the rcu_barrier() invocation's callbacks to >> > be leaked. >> > >> > o The second thread's rcu_barrier() invocation never returns >> > resulting in a hang. >> > >> > This commit therefore changes call_rcu_data_free() to acquire >> > call_rcu_mutex before checking the call_rcu_data structure for callbacks. >> > In the case where there are no callbacks, call_rcu_mutex is held across >> > both the check and the removal from call_rcu_data_list, thus preventing >> > rcu_barrier() from adding a callback in the meantime. In the case where >> > there are callbacks, call_rcu_mutex must be momentarily dropped across >> > the call to get_default_call_rcu_data(), which can itself acquire >> > call_rcu_mutex. This momentary drop is not a problem because any >> > callbacks that rcu_barrier() might queue during that period of time will >> > be moved to the default call_rcu_data structure, and the lock will be >> > held across the full time including moving those callbacks and removing >> > the call_rcu_data structure that was passed into call_rcu_data_free() >> > from call_rcu_data_list. >> > >> > With this fix, a several-hundred-CPU test successfully completes more >> > than 5,000 executions. Without this fix, it fails within a few tens >> > of executions. Although the failures happen more quickly on larger >> > systems, in theory this could happen on a single-CPU system, courtesy >> > of preemption. >> >> I agree with this fix, will merge in liburcu master, stable-0.12, and >> stable-2.11. >> Out of curiosity, which test is hanging ? Is it a test which is part of the >> liburcu >> tree or some out-of-tree test ? I wonder why we did not catch it in our CI >> [1]. > > The hung test was from perfbook [1] in the CodeSamples/datastruct/hash > directory. A repeat-by is as follows: > > # Have userspace RCU preinstalled as you wish. > git clone git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git > cd CodeSamples > make pthreads > cd datastruct/hash > make > time for ((i = 0; i < 2000; i++)); do echo $i; ./hash_bkt_rcu --schroedinger > --nreaders 444 --nupdaters 4 --duration 1000 --updatewait 1 --nbuckets 262144 > --elems/writer 65536; done > > This normally hangs within a few tens of iterations. With this patch, > the passes more than 6,000 iterations. > > I have smaller tests that produce this same hang on my 12-CPU laptop, > but with much lower probability. Here is one example that did hang on > my laptop, and which could be placed into a similar bash loop as above: > > hash_bkt_rcu --schroedinger --nreaders 10 --nupdaters 2 --duration 1000 > --updatewait 1 --nbuckets 8192 --elems/writer 4096 > > But I don't have a good estimate of the hang probability, except a > suspicion that it is lower than would be convenient for a CI test. > Attaching to the hung process using gdb did confirm the type of hang, > however. > > It might be possible to create a focused test that races rcu_barrier() > against thread exit, where threads are created and exit repeatedly, > and make a per-thread call_rcu() worker in the meantime.. > > Thoughts?
We'll try to come up with a narrowed-down test-case which reproduce the issue. We may get some inspiration from your test-case. I'll ask Michael Jeanson to add this to our backlog. Thanks, Mathieu > > Thanx, Paul > > [1] git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git > >> Thanks, >> >> Mathieu >> >> [1] https://ci.lttng.org/view/Liburcu/ >> >> > >> > Signed-off-by: Paul E. McKenney <paul...@kernel.org> >> > Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> >> > Cc: Stephen Hemminger <step...@networkplumber.org> >> > Cc: Alan Stern <st...@rowland.harvard.edu> >> > Cc: Lai Jiangshan <jiangshan...@gmail.com> >> > Cc: <lttng-dev@lists.lttng.org> >> > Cc: <linux-ker...@vger.kernel.org> >> > >> > --- >> > >> > urcu-call-rcu-impl.h | 7 +++++-- >> > 1 file changed, 5 insertions(+), 2 deletions(-) >> > >> > diff --git a/src/urcu-call-rcu-impl.h b/src/urcu-call-rcu-impl.h >> > index b6ec6ba..18fd65a 100644 >> > --- a/src/urcu-call-rcu-impl.h >> > +++ b/src/urcu-call-rcu-impl.h >> > @@ -772,9 +772,13 @@ void call_rcu_data_free(struct call_rcu_data *crdp) >> > while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == >> > 0) >> > (void) poll(NULL, 0, 1); >> > } >> > + call_rcu_lock(&call_rcu_mutex); >> > if (!cds_wfcq_empty(&crdp->cbs_head, &crdp->cbs_tail)) { >> > - /* Create default call rcu data if need be */ >> > + call_rcu_unlock(&call_rcu_mutex); >> > + /* Create default call rcu data if need be. */ >> > + /* CBs queued here will be handed to the default list. */ >> > (void) get_default_call_rcu_data(); >> > + call_rcu_lock(&call_rcu_mutex); >> > __cds_wfcq_splice_blocking(&default_call_rcu_data->cbs_head, >> > &default_call_rcu_data->cbs_tail, >> > &crdp->cbs_head, &crdp->cbs_tail); >> > @@ -783,7 +787,6 @@ void call_rcu_data_free(struct call_rcu_data *crdp) >> > wake_call_rcu_thread(default_call_rcu_data); >> > } >> > >> > - call_rcu_lock(&call_rcu_mutex); >> > cds_list_del(&crdp->list); >> > call_rcu_unlock(&call_rcu_mutex); >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. > > http://www.efficios.com -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
